1. Field of the Invention
This invention relates to apparatus and method for collecting and time-averaging visual information carried within "noisy" video signals, and more particularly, relates to apparatus and method for collecting and time-averaging visual information carried within "noisy" video signals which are generated by scanned particle-beam instruments such as the scanning electron microscope ("SEM"). The invention is generally applicable to any problem of recovering a spatially stationary image signal buried within large amounts of superimposed random noise, such as that often found in low-current or low-light level imaging detectors.
2. Description of the Prior Art
Signal-to-noise ratio enhancement of SEM images is typically performed (usually in conjunction with scan rate conversion) by an electronic system such as that shown in block form in FIG. 1. The incoming (noisy) video signal is periodically sampled and converted to a series of digital intensity values, which are stored sequentially in a memory array, individually addressed by the co-ordinate of their position in the image. With each new video frame cycle, the stored values are fed back to computational circuitry which performs a "running average" on a pixel by pixel basis, summing new data collected at a given pixel with data previously stored for that pixel. The old and new data is mixed in predetermined proportions, then renormalized (to avoid overflow) and stored back in the same memory location.
The image being processed may be viewed concurrently by feeding the current pixel data in sequence to the output DAC at the desired display frequency. As successive frames are collected and processed the stationary image tends to improve as the random noise component averages out to zero. By varying the proportions of new and stored data, it is possible to control the rate of signal-to-noise ratio improvement according to the incoming signal quality.
This process is a type of digital signal processing variously known as temporal filtering, digital integration, or frame averaging. A special case of the temporal filter is the progressive-ratio or Kalman filter, in which the "mixing ratio" is a linear function of frame count. The Kalman filter (or a near approximation to it) is widely used for SEM imaging as it collects the maximal amount of information per unit time while maintaining an algebraically normalized output at all times.
As the signal-to-noise ratio of the input increases, however, the Kalman filter suffers (as do all fixed-ratio filters) from a "shrinkage" of the output amplitude as the noise component is averaged out. Increasing the input amplitude cannot compensate for this as the input dynamic range is limited by the analog-to-digital converter.